Linear motor outperforms steam-piston catapults

Hudson, MA--In a large shed off a back road in this rural Massachusetts town, engineers are developing the world's highest-thrust linear motor. For Naval aviation, it may mean the end of the age of steam.

By 2005, naval architects may begin replacing the enormous steam catapults on aircraft carriers with EMALS--the electromagnetic aircraft launch system. "We're hoping to replace heavy machinery with electronics," says Wolfgang Schlegel, EMALS program manager at Kaman Electromagnetics Corp. Under a Navy contract, the program has reached its Critical Component Demonstration phase, with daily firing of a test rig to prove the reliability and controllability of its constituent parts: a control system, a cycloconverter power conditioner, and the linear motor itself.

Developed originally for powering electromagnetic rail guns, the pulse alternator comprises a massive flywheel rotating at 6,000 rpm. At a signal, the test article delivers as much as 2 MW of electrical power to drive the linear motor. The cycloconverter transforms the high-frequency ac power to lower-frequency ac for the motor's use. Finally, the linear motor consists of modular arrays of NdBFe magnets and a full scale stator segment. In use aboard an aircraft carrier, a turbine engine would provide input power for an 8-MW pulse alternator, allowing 200-kt catapult firings of 100,000-lb aircraft every 45 seconds.

The test fixture consists of a 450-lb carriage riding on parallel rails. Centered between the rails at their midpoint stands a series of three modular stator segments. An array of encapsulated magnets affixed to the carriage interacts with the stator as the carriage passes by. At one end of the fixture, a large pneumatic ram pushes the carriage past the stator, where it's accelerated further. At the opposite end, a group of hydraulic dashpots and/or sacrificial foam blocks decelerate the carriage at the end of a run.

By throttling the ram to deliver the carriage to the linear motor segments at various speeds, engineers emulate every phase of a carrier launch without the expense of building a full-scale catapult. A line of proximity sensors alongside the rails adjacent to the stator records the system's precision in controlling acceleration. So far, says Schlegel, "We've got closed-loop control and response time--we're working in microseconds."

One indicator of the power of the system: 4 ft of stator area accelerates the 450-lb carriage to 7 kts. The test series is continuing, with 200-kt shots the final goal.

If approved for production, EMALS would bring a host of improvements to ship design and Naval aviation. With a maximum design thrust of 290,000 lbs, EMALS offers 28% greater launching capability than steam catapults. This improvement may allow flight operations regardless of whether the carrier is turned into the wind. That thrust is fully controllable, allowing compensation for wind gusts and more precise matching of thrust to aircraft weight than is possible with steam. Closed-loop control of thrust will produce less wear and tear on pilots as well as aircraft. By one study, the reduced stress from "soft starts" could extend airframe life as much as 31%.

Prototype magnet and stator sections have already passed testing for longevity in harsh, saltwater environments. Schlegel says that, since EMALS will be constructed in modular sections, it should achieve economies of scale in manufacturing, as well as granting designers greater flexibility in putting assisted-launch capability onto unconventional platforms. For example, ski-jump-style carriers could be given the ability to launch heavier aircraft or heavily laden STOVL aircraft could be launched from portable EMALS erected on small islands or clearings.

Because the system is powered by a turbine engine, it's independent of the carrier's nuclear or oil-fired power plant, thus increasing tactical flexibility. The system converts about 70% of input energy into aircraft kinetic energy compared to a steam catapult's 6% efficiency. Moreover, Kaman engineers envision accelerating an aircraft with only the first 2/3 of the modular stator segments. The remaining third will decelerate the shuttle once the plane becomes airborne. Deceleration will recover some of the input energy and further boost system efficiency.

In the future, engineers hope to develop an electric arrestor system, replacing the currently used hydraulic dampers with high-capacity eddy-current brakes. In that case, decelerating incoming aircraft could help accelerate others taking off. Until then, the short-term payoff is that four EMALS launchers will take up no more space than that needed by current steam systems, but will cut topside weight by 1,000 tons--a naval architect's dream.

Additional details...Contact Peter Mattila, Kaman Electromagnetics Corp., 2 Fox Rd., Hudson, MA 01749, (508) 562-2933.